Support Lambda PassThrough trace header propagation

Issue

Lambda is implementing PassThrough mode which allows customers to send traces through instrumented lambda functions without those functions being traced. The way this is done is by changing the trace header structure from Root=...;Parent=...;Sampled=1 in the active tracing case to Root=... in the PassThrough case. As such, our SDK needs to be updated to handle this.

Changes

Update logic to use no-op segment if parent and sampled are missing (ie. PassThrough mode)
- Had to use SuppressWarnings("nullness") because TraceHeader.getRootTraceId() returns an @Nullable TraceId but Segment.noOp() expects a @NonNull TraceId
Update all trace header propagations to only propagate Root if we're not actively sampling
- Not propagating Parent and Sampled is fine because Parent will be re-generated in the next traced context and missing Sampled will be treated as not actively sampling anyways
- We can't propagate only one of these, it needs to be either both or neither
Updated unit tests

Testing

Ran many extensive tests using v2.16.0, v2.17.0, and these changes to compare the outcomes. In these tests I configured 3 lambda functions to interact with each other ALL instrumented with the new changes:

Lambda A: Calls Lambda B
Lambda B: Calls Lambda C
Lambda C: Calls GetAccountSettings

The trace headers in the below cases are from logging System.getenv("_X_AMZN_TRACE_ID") in the lambda functions themselves

The following are some important cases that prove the functionality of the code change

Case 1.a: Lambda A Active → Lambda B PassThrough → Lambda C Active

Lineage is NOT propagated or received by any of these functions
Lambda B has an inferred node due to the invocation in Lambda A which is actively traced, this is expected behaviour and not controlled by the SDK
Lambda B function is being appropriately suppressed aside from the previous note
Lambda C invocation is appropriately connected to the Lambda A function

A: Root=1-66abd797-167a78c02dcfeb4551890716;Parent=ad075ec25766d3f9;Sampled=1;Lineage=456f6a97:0
B: Root=1-66abd797-167a78c02dcfeb4551890716;Parent=57696313aea6f4a2;Sampled=1;Lineage=8408dcb0:0
C: Root=1-66abd797-167a78c02dcfeb4551890716;Parent=710d6393cf68bccc;Sampled=1;Lineage=7d2b6fd0:0

Case 1.b: Same thing but Lambda A Active sets `Sampled=0`

Lineage is NOT propagated or received by any of the functions
No trace is generated
Lambda A, B, and C all have Sampled=0
Lambda C appropriately changes Parent as expected, as it is actively traced, but still maintains the Sampled=0

A: Root=1-66b32340-6a4114ff44ca3e5240b1ac5b;Parent=63033870537134cd;Sampled=0;Lineage=456f6a97:0
B: Root=1-66b32340-6a4114ff44ca3e5240b1ac5b;Parent=63033870537134cd;Sampled=0;Lineage=8408dcb0:0
C: Root=1-66b32340-6a4114ff44ca3e5240b1ac5b;Parent=71d11ee442af5530;Sampled=0;Lineage=7d2b6fd0:0

Case 2: Lambda A PassThrough → Lambda B Active → Lambda C PassThrough

Lineage is NOT propagated or received by any of these functions
Lambda B is the start of the trace as expected
Lambda C invocation is still attached to Lambda B as an inferred node (similar to case 1)
Lambda C's GetAccountSettings call IS TRACED; this is expected behaviour because we entered Active tracing mode in Lambda B and this is meant to propagate to downstream services. If the downstream service has its own version of passive tracing/X-Ray integration, it should be configured as such, but PassThrough functionality should only apply to Lambda functions and not the downstream calls they trigger unless the entire pathway has been in PassThrough
```
A: Root=1-66b3b030-7155e4f41eac6ff601dfaf63;Lineage=456f6a97:0
B: Root=1-66b3b030-7155e4f41eac6ff601dfaf63;Parent=ad139ed6effb18d2;Sampled=1;Lineage=8408dcb0:0
C: Root=1-66b3b030-7155e4f41eac6ff601dfaf63;Parent=50262dfb251d8754;Sampled=1;Lineage=7d2b6fd0:0
```

Case 3.a: Calling Lambda B PassThrough → Lambda C PassThrough

Lineage not propagated
No trace, proves statement above that a complete trace entirely in PassThrough mode will not trace the final GetAccountSettings call since we never entered Active tracing mode.
```
B: Root=1-66abfc06-3cbd74727bb2f86a6a772af2;Lineage=8408dcb0:0
C: Root=1-66abfc06-3cbd74727bb2f86a6a772af2;Lineage=7d2b6fd0:0
```

Case 3.b: Calling Lambda B Active → Lambda C Active

Lineage not propagated

Complete and correct trace

B: Root=1-66abfd06-5e95193a3dcfa6a25cec5b30;Parent=11c07dc8ed123be7;Sampled=1;Lineage=8408dcb0:0
C: Root=1-66abfd06-5e95193a3dcfa6a25cec5b30;Parent=613a4ff907527f30;Sampled=1;Lineage=7d2b6fd0:0

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

aws / aws-xray-sdk-java