allow ignoring comments in migration checksum

[siosw]

I have found these related issues/pull requests

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Description

currently migration checksums include comments, this makes it hard to change them once a migration has been applied. sometimes this is nice to add more context later etc.

Prefered solution

add some logic filtering out comments here:

sqlx/sqlx-core/src/migrate/source.rs

Line 118 in 764ae2f

let sql = fs::read_to_string(&entry_path).map_err(|e| ResolveError {

Is this a breaking change? Why or why not?

this would break existing migration checksums so it should be behind a feature flag

[abonander]

This could be implemented on top of #3383

[siosw]

@abonander would love to take a stab at a PR for this. I think we could add a ignore_migration_comments field to the sqlx.toml which defaults to false. if set we ignore comments in the checksum.

wdyt?