Define lower bounds for header limits

[dyladan]

Fixes #65

1. Remove the current upper bound limits (8192 total bytes, 180 entries, 4096 bytes per entry)
2. Define the following minimum propagation requirements:
   1. At least 64 entries MUST be propagated
   2. At least 8192 bytes including delimiters MUST be propagated

Provides the following benefits:

• A user can reliably assume 64 entries with length of 127 characters or less will be propagated
• A platform vendor with additional resources can enable much higher limits for their users as these are only minimum requirements, enabling use cases for their users which use more than 64 entries or which use longer entries.
• A platform vendor with very strict processing requirements can track changes with a 64-bit bit set and allocate a fixed-length buffer for serialization.

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Preview | Diff

[dyladan]

Clarified the wording at @kalyanaj suggestion to be clear that if the minimum is not met, all entries should be propagated.

Clarified wording at @SergeyKanzhelev suggestion to be clear that the limit applies to the total of all baggage headers if there are multiple headers.

[SergeyKanzhelev]

As discussed today, merging