fix: Add TextClause to alter_column's server_default fields.

[DanCardin]

Description

Given sqlalchemy models with server_defaults, alembic may autogenerate existing_server_default=sa.text("..."), which fails typechecking on the resultant migration because TextClause is not currently a valid annotated type.

I'm just not sure whether there's a more general type that would make more sense or not. just that given what alembic itself is generating, this should presumably be allowed.

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • please include the issue number, and create an issue if none exists, which
    must include a complete example of the issue. one line code fixes without an
    issue and demonstration will not be accepted.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests. one line code fixes without tests will not be accepted.
• A new feature implementation
  • please include the issue number, and create an issue if none exists, which must
    include a complete example of how the feature would look.
  • Please include: Fixes: #<issue number> in the commit message
  • please include tests.

Have a nice day!