T7596: Support independent configuration of echo receive-interval and echo transmit-interval in BFD

[factor2431]

Change summary

1. Support independent configuration of echo receive-interval and echo transmit-interval in BFD
2. The old echo-interval will be migrated to echo-receive and echo-transmit with the same value

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] 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/T7596

Related PR(s)

https://github.com/vyos/vyos-documentation/pull/1652

How to test / Smoketest result

Checklist:

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