Add new method for custom formatter handling

[ivuorio]

Adding a method for setting new formatter into the native formatter handling. This does not change how the existing solutions work, and thus should not cause any backwards compatibility issue. But also allows user to implement multiple custom formatters and allows using them in datasets according to their name just like the builtin formatters. This solves both problems from the issue #290 as the the custom formatter is handled just like builtin ones.

Adding:

• New method for adding custom formatters
• Updating the documentation about the custom formatter using

Testing:

• Manually tested with multiple datasets in YourTTS model training. Works with both compute embeddings and dataset loader.
• Test pass.

ToDo:

• Add unit tests for the new method
• Add unit tests for custom formatter