feat(core): allow fine-grained control over em.upsert()

[B4nan]

Adds 4 new options to em.upsert():

• onConflictFields?: (keyof T)[] to control the conflict clause
• onConflictAction?: 'ignore' | 'merge' used ignore and merge as that is how the QB methods are called
• onConflictMergeFields?: (keyof T)[] to control the merge clause
• onConflictExcludeFields?: (keyof T)[] to omit fields from the merge clause

Closes #4325
Closes #4602

[codecov]

Codecov Report

Patch coverage is 99.17% of modified lines.

❗ Current head 76d493d differs from pull request most recent head c84c034. Consider uploading reports for the commit c84c034 to get more accurate results

Files Changed	Coverage
packages/core/src/drivers/IDatabaseDriver.ts	ø
packages/core/src/utils/upsert-utils.ts	94.44%
packages/core/src/EntityManager.ts	100.00%
...ckages/core/src/unit-of-work/ChangeSetPersister.ts	100.00%
packages/core/src/utils/index.ts	100.00%
packages/knex/src/AbstractSqlDriver.ts	100.00%
packages/mongodb/src/MongoConnection.ts	100.00%
packages/mongodb/src/MongoDriver.ts	100.00%

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