feat(managers/custom): introduce customType field

[RahulGautamSingh]

Changes

• add customType field to custom manager config
• Rename RegExManager to CustomManager
• Add migration and validation code for customType
• Update regexManager presets to include customType: 'regex'

Context

Part of #19066

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[rarkins]

If the customType default is regex, why do we need to migrate configs to include it?

[RahulGautamSingh]

If the customType default is regex, why do we need to migrate configs to include it?

It's not default regex atm

[rarkins]

@viceice @secustor we have a choice between these:

1. Default to regex manager

• This makes regex the "preferred" custom manager forever (similar to platform=github)
• No migration necessary for existing configs

2. Require regex manager

• Requires migration PRs for every existing config
• Still makes regex the preferred custom manager because anyone leaving it off gets it migrated

I think I'd prefer to keep it simple with (1) unless you think there's too much confusion?

[secustor]

I would prefer 2 as it is more verbose and signals to new or users with limited exposure that there are more options.

This is IMO not comparable to platform=github as this is a self-hosted option and not an actual option an user would interact much with.
Thinking here of bigger deployments.

[renovate-release]

🎉 This PR is included in version 36.56.0 🎉

The release is available on:

• GitHub release
• 36.56.0

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