Add the concept of "validation_mode"

[joesaunderson]

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Documented?	yes
Fixed tickets	N/A
License	MIT

Adds the concept of validation_mode at the default config level.

[Vincz]

Excellent @joesaunderson! You are on the right track! Do you think it would be possible to also be able to manage the validation mode on the object itself (if it's too complicated, we'll try to implement in a future release).
If you can also add the test (with the same object applying both modes and validate the output).

[TomasLudvik]

Hello @joesaunderson and @Vincz , is it possible to move his PR forward as it solves problem introduced in #1184 ? We are locked on version 1.3.2 and are waiting for this to be fixed. Thank you.

[joesaunderson]

I won't have the time I'm afraid, but someone can take this base and make the changes @Vincz suggests.

[murtukov]

@TomasLudvik #1184 has been rolled back in #1214

[murtukov]

Closing this PR because it was opened based on the incorrect assumption that the validator asserts optional fields. In reality, if an optional field is omitted, it isn’t validated unless it’s explicitly marked with Assert\NotBlank.