feat(GlFormInput): Add size prop

What does this MR do?

feat(GlFormInput): Add size prop

This implements form input sizing according to our design specifications.

These are the available sizes:

Name	(Max) Width
xs	80px
sm	160px
md	240px
lg	320px
xl	560px
(unset or null)	n/a (the default, which is how inputs currently behave)

Addresses #911 (closed).

Integration MR

I'm not sure this is necessary, but I'd like to check at least nothing breaks

The integration MR reveals a fair amount of breakage, due to snapshot tests on shallow mounted components with form input children. I'm considering attempting a different strategy, where the default size is null, such that the size prop doesn't get added to the snapshot.

I've changed the implementation now so that consuming application test suites shouldn't be broken by this.

gitlab!37968 (closed)

Does this MR meet the acceptance criteria?

Conformity

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