OnlineDDL: request_context/migration_context

[shlomi-noach]

We add a context to an OnlineDDL request:

• who made the request? (e.g. vtctl, vtctld/api, vtgate)
• context parameters? (e.g. session ID, or unique ID for vtctl execution)

This context (not to confuse with golang context.Context) can be later used to group and associate migrations. For example, knowing that a few migration requests all came from the same vtctl call, means we should probably prioritize to run them together.

[shlomi-noach]

Still TODO: pass request_by and request_param in VExec query. I struggle to figure this one out, I think we're shooting ourselves in the foot with VExec: how do you add new columns to a Vexec query, and at the same time make it backwards compatible? Specifically, if a user upgrades their vtctld or vtgate servers, but not their vttablet servers, then the addition of new columns is rejected by vttablet. VExec creates coupling where normal golang mechanisms already provide the decoupling framework to deal with missing/default values.

[sougou]

Since VExec is not GA yet, it doesn't have to be backward compatible.
For grouping: we've been using unique workflow names for vreplication, and that has worked out well so far. We can see if the same approach works here.
As for prioritization, it probably needs to be enforced by a policy because different people may want different things.

[shlomi-noach]

Converged into a single param, migration_context or request_context, that is an arbitrary string.

[shlomi-noach]

Because Online DDL is still experimental, and because I need this functionality for some internal POC, I'm just modifying the schema without beackwards compatibility.

[shlomi-noach]

ready for review

[deepthi]

LGTM