Conditional branching in configs?

We often have transforms which need to vary, say, sequence to sequence.

For example: say most of the show converts to `default_log` to apply a client grade.. except one sequence which needs `another_log`.

We currently implement this using the "lut_search_path fallback" pattern. As in, we have a fallback folder containing a fixed filename (usually symlinked to the desired LUT), and create folders for the sequences requiring overrides:
```
/path/to/luts/pershot-fallback:
per_shot.lut -> /path/to/default_log.lut
/path/to/luts/pershot-ZZZ:
per_shot.lut -> /path/to/another_log.lut
```

then `lut_search_path` contains something like:
```yaml
lut_search_path: /path/to/luts/pershot-$SEQ:/path/to/luts/pershot-fallback
```

So then when the colorspace definition (or look etc) references the pershot.lut:

```
! {src: pershot.lut}
```
..it will either find the LUT in `pershot-ZZZ` if $SEQ is set to that, and for all others values of $SEQ it will end up at the `pershot-fallback` folder.

While this approach works, it is far from ideal. Main issues are:

1. It's convoluted to setup, requiring two search-path entries per LUT which needs switched and at least the fallback LUT created on disk (and since it must have a generic filename like `pershot`, the config no longer inherently describes what this transform actually does)
2. The config is somewhat opaque - to someone not familiar with the pattern, it's unclear what is going on.
3. Switching more complex transforms is not practical. If there is multiple steps in the transform which need to vary, this becomes a mess (separate `lut_search_path` entries, dummy transforms etc)
4. Switching transforms not based on files will not work (e.g MatrixTransform, ExponentTransform, ColorSpaceTransform, future things like the possible builtin-transforms), short of baking them to LUT's

One idea which was discussed a long time ago was to essentially take a ColorSpace definition config snippet and write this to a file - kind of like an "OCIO powered mega-LUT" file - then reference this in your config using the fallback pattern. This solves points 3 and 4, but not so much the other two points. It lets you switch multiple transforms, but still convoluted/opaque.

A different way this could be approached is to have a "meta-transform" (along the lines of the `GroupTransform`). This meta-transform would have a condition (e.g "is the `SEQ` context variable set to a specific value?"), then a transform to use if the condition is met, and and "else" transform

One possible way of encoding this might be (stealing inspiration from Shotgun API's filter conditions):

```yaml
to_reference: !
children:
- !
context_var: SHOT
operation: in # also things like equals, starts_with, not in etc?
condition_value: ["abc123", "abc321", "def1212", "fed2323"]
transform_true:
! {src: another_log.lut}
transform_false:
!
children:
- ! {src: default_log.lut}
- ...
```

This would encode the psuedocode logic of:

```
if $SHOT in ('abc123', 'abc321', or ...):
transform = FileTransform("another_log.lut")
else:
transform = FileTransform("default_log.lut")
```

These `ConditionalTransforms` could be nested inside GroupTransforms or used at top-level of a ColorSpace defintion. It could also support branching from things other than context-vars (potentially things like if it's a CPU or GPU code path, host names, host applications etc etc)

While I'm not 100% happy with the above mockup config syntax, I feel the general approach of encoding these conditional explicitly in the config file could be very valuable. Thoughts?

この issue では実装ファイルやテストが指定されていません。まず既存の GroupTransform とここで説明されている設定構文を確認し、その後、サポートされる条件演算、ネストの動作、フォールバックのセマンティクスを明確にしてから、完了の定義を決めてください。

索引モデルが issue の本文から書いたものです。