Online DDL: internal support for eta_seconds

[shlomi-noach]

Description

Objective: make both migration progress and ETA accessible to the user.

Initial commits:
This PR adds the column eta_seconds to _vt.schema_migrations and provides the following initial support:

• Initializes with negative value (negative means "N/A", unknown)
• accepts a eta=? query param on /schema-migration/report-status (which is called on gh-ost and pt-osc migrations)
• reports GH_OST_ETA_SECONDS in gh-ost status hook

TODO/related:

• Make progress pct and ETA available to the user; probably moving away from VExec
• Provide a SQL interface

Related Issue(s)

• Depends on this gh-ost build: hooks: reporting GH_OST_ETA_SECONDS. ETA as part of migration context openark/gh-ost#17
• Tracking: Online DDL: tracking issue #6926
• • suggested syntax: Suggestions for online DDL query syntax #7604

Checklist

• Should this PR be backported?
• Tests were added or are not required
• Documentation was added or is not required

Deployment Notes

Impacted Areas in Vitess

Components that this PR will affect:

• Query Serving
• VReplication
• Cluster Management
• Build/CI
• VTAdmin

[shlomi-noach]

This is now ready for review. This PR only implements internal support for eta_seconds and right now only gh-ost populates that value.

Future PRs will:

• make this value accessible via SQL interface
• implement progress/ETA in vreplication based migrations

[shlomi-noach]

cc @piki

[rohit-nayak-ps]

lgtm