srv6: T6977: add srv6 encapsulation source-address

[jvoss]

Change summary

Add configuration option for setting the source address of the outer encapsulation IPv6 header on SRv6 traffic:
https://docs.frrouting.org/en/latest/zebra.html#clicmd-source-address-X-X-X-X

protocols 
 segment-routing {
  srv6 {
    encapsulation {
      source-address "2001:db8::1"
    }
  }
 }
}

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T6977

Related PR(s)

• T6977: FRR patch for 'no encapsulation' command vyos-build#1115

How to test / Smoketest result

$ /usr/libexec/vyos/tests/smoke/cli/test_protocols_segment-routing.py
test_srv6 (__main__.TestProtocolsSegmentRouting.test_srv6) ... ok
test_srv6_encap_source_addr (__main__.TestProtocolsSegmentRouting.test_srv6_encap_source_addr) ... ok
test_srv6_sysctl (__main__.TestProtocolsSegmentRouting.test_srv6_sysctl) ... ok

----------------------------------------------------------------------
Ran 3 tests in 9.384s

OK

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[github-actions]

👍
No issues in PR Title / Commit Title

[jvoss]

I have read the CLA Document and I hereby sign the CLA

[c-po]

PR review is currently paused. We try to advise with FRR team

[c-po]

Squashed in the requested CLI help string from @dmbaturin and rebased to current/HEAD to get a good CI re-run.

[github-actions]

CI integration 👍 passed!

Details

CI logs

• CLI Smoketests 👍 passed
• CLI Smoketests (interfaces only) 👍 passed
• Config tests 👍 passed
• RAID1 tests 👍 passed
• CLI Smoketests VPP 👍 passed
• Config tests VPP 👍 passed
• TPM tests 👍 passed

[hedrok]

Approve: read code, launched smoke test, manually ran:

configure
set protocols segment-routing srv6 encapsulation source-address 3fff:aaa::1
set protocols segment-routing interface eth3
commit
# Checked that frr configuration has this source  address
delete protocols segment-routing
commit
# Checked that FRR configuration is cleaned

[sever-sever]

Add source-address for srv6

[c-po]

CLI extension looks good. All requested changes done. Manually tested by @hedrok