tls-outbounds: make spiffe ids field as optional field

[jronak]

Earlier, tls outbound required at least one spiffe id as we wanted to use it to match the server id. Since managing server spiffe id across clients causes issues during the migration, this PR makes spiffe ids an optional field. Outbounds will accept any server certificate issued by the internal CA with any spiffe id if no spiffe ids configuration is provided.

• [X] Description and context for reviewers: one partner, one stranger
• [ ] Docs (package doc)
• [X] Entry in CHANGELOG.md

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (f345554) 85.36% compared to head (05f91cd) 85.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #2216 +/- ## ======================================= Coverage 85.36% 85.36% ======================================= Files 270 270 Lines 15479 15473 -6 ======================================= - Hits 13214 13209 -5 + Misses 1844 1843 -1 Partials 421 421 ``` | [Impacted Files](https://app.codecov.io/gh/yarpc/yarpc-go/pull/2216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc) | Coverage Δ | | |---|---|---| | [transport/grpc/config.go](https://app.codecov.io/gh/yarpc/yarpc-go/pull/2216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc#diff-dHJhbnNwb3J0L2dycGMvY29uZmlnLmdv) | `97.26% <ø> (-0.04%)` | :arrow_down: | | [transport/http/config.go](https://app.codecov.io/gh/yarpc/yarpc-go/pull/2216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc#diff-dHJhbnNwb3J0L2h0dHAvY29uZmlnLmdv) | `95.65% <ø> (-0.10%)` | :arrow_down: | | [transport/tchannel/config.go](https://app.codecov.io/gh/yarpc/yarpc-go/pull/2216?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc#diff-dHJhbnNwb3J0L3RjaGFubmVsL2NvbmZpZy5nbw==) | `93.65% <ø> (-0.20%)` | :arrow_down: | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/yarpc/yarpc-go/pull/2216/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=yarpc)

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[lambdai]

Can you also add into the description: When the empty spiffie id config is provided, what new server config is accepted

My interpretation is that it allow server providing any legit spiffie id but still need to be a legit spiffied id format

[jronak]

Can you also add into the description: When the empty spiffie id config is provided, what new server config is accepted

My interpretation is that it allow server providing any legit spiffie id but still need to be a legit spiffied id format

Updated, thanks!