[V3] Allow custom adapters

[ineshbose]

Is your feature request related to a problem? Please describe

Since V3 is SQL based and provides presets for what DB, it would be quite great if devs can provide their own adapters (especially when someone may be using Drizzle, Prisma, etc). This also enables a lot more control over how collections can integrate into existing databases.

Describe the solution you'd like

I definitely do not propose providing custom adapter implementation within the module as it would require lot of maintenance. Instead, if there is a file like content.adapter.ts, we load that into #content/adapter.

A couple of things we'd need to do additionally:

1. Expose createDatabaseAdapter.
2. Do you think we can do away with adapter.exec? We should rather follow this format for adapters: https://github.com/better-auth/better-auth/blob/9e56ea2e325b916cbbca24f0478bf91484bf7668/packages/better-auth/src/adapters/drizzle-adapter/drizzle-adapter.ts#L248 https://github.com/better-auth/better-auth/blob/9e56ea2e325b916cbbca24f0478bf91484bf7668/packages/better-auth/src/types/adapter.ts#L26
3. If we keep exec, we may need to allow collection configuration to specify tableName (or we provide instructions that devs configure their ORM to match __content_${name}, but that defeats the purpose)

Describe alternatives you've considered

Additional context

Willing to raise a PR -- need your thoughts first!