Add source generator for custom transforms

[smoogipoo]

Prereqs:

• Refactor source generation for abstraction #5760

Continuing on with the efforts to remove reflection from our codebase.

This is a WIP PR, which works but has a few things to clean up/fix (interface support and tests). I'm making this PR ahead of time (pun intended) for if there are any opinions to be had.

Code-gen can be previewed here:

• Source: https://github.com/smoogipoo/osu-framework/blob/5c30976ab5cc8a75970f2d54a4ea22d29967a9c8/osu.Framework.SourceGeneration.Tests/Resources/Transforms/Test/Sources/Test.txt
• Generated: https://github.com/smoogipoo/osu-framework/blob/5c30976ab5cc8a75970f2d54a4ea22d29967a9c8/osu.Framework.SourceGeneration.Tests/Resources/Transforms/Test/Generated/g_Test_Transforms.txt
• Usages:
  • https://github.com/ppy/osu-framework/compare/master...smoogipoo:custom-transform-sgen?expand=1#diff-f2d5e092f61fa20c96dc82a7fcef41029424761ff8e190b7f9346c0c6a79fbc7
  • https://github.com/ppy/osu-framework/compare/master...smoogipoo:custom-transform-sgen?expand=1#diff-f3c8520a6c69f0098cb94cb2a9c6cb6a162e526a4edf9df0a8b86919a3b708a8

Before:

Method	Mean	Error	StdDev	Allocated
UpdateTransformsWithNonePresent	4.884 us	0.0110 us	0.0103 us	-
UpdateTransformsWithManyPresent	366.357 us	0.7482 us	0.5842 us	-

After:

Method	Mean	Error	StdDev	Allocated
UpdateTransformsWithNonePresent	7.063 us	0.0227 us	0.0212 us	-
UpdateTransformsWithManyPresent	303.062 us	0.7666 us	0.7171 us	-

Before:

Method	Mean	Error	StdDev	Gen 0	Allocated
CreateSingleBlank	9.897 ns	0.1502 ns	0.1254 ns	0.0011	96 B
CreateSequenceThenClearAfter	5,834.537 ns	70.3694 ns	65.8235 ns	0.0534	4,688 B
Expiry	31,483.607 ns	611.2051 ns	541.8174 ns	-	1,208 B
CreateSequenceWithDefaultEasing	1,691.055 ns	33.5365 ns	34.4395 ns	0.0134	1,200 B
ApplySequenceWithDefaultEasing	501.340 ns	8.3260 ns	7.7882 ns	0.0029	272 B
CreateSequenceWithValueEasing	1,648.158 ns	21.6247 ns	20.2277 ns	0.0134	1,200 B
ApplySequenceWithValueEasing	482.865 ns	9.1271 ns	8.9640 ns	0.0029	272 B
CreateSequenceWithReferenceEasing	1,617.588 ns	31.4736 ns	36.2450 ns	0.0134	1,272 B
ApplySequenceWithReferenceEasing	494.782 ns	9.5572 ns	9.3864 ns	0.0029	296 B

After:

Method	Mean	Error	StdDev	Gen 0	Allocated
CreateSingleBlank	10.05 ns	0.149 ns	0.117 ns	0.0011	96 B
CreateSequenceThenClearAfter	5,002.43 ns	93.678 ns	87.626 ns	0.0534	4,776 B
Expiry	31,206.24 ns	370.593 ns	346.653 ns	-	1,232 B
CreateSequenceWithDefaultEasing	1,424.96 ns	23.495 ns	21.977 ns	0.0134	1,224 B
ApplySequenceWithDefaultEasing	444.87 ns	6.703 ns	5.942 ns	0.0033	280 B
CreateSequenceWithValueEasing	1,458.25 ns	28.271 ns	31.423 ns	0.0134	1,224 B
ApplySequenceWithValueEasing	440.80 ns	8.529 ns	11.956 ns	0.0033	280 B
CreateSequenceWithReferenceEasing	1,511.23 ns	10.894 ns	9.657 ns	0.0153	1,296 B
ApplySequenceWithReferenceEasing	450.94 ns	8.093 ns	7.570 ns	0.0033	304 B