[Impeller] Add 2D texture array support (kTexture2DArray) (#188778)

Part of flutter/flutter#188776.

Adds a 2D texture array texture type to Impeller
(`TextureType::kTexture2DArray`, sampled as `sampler2DArray`), with the
layer count carried by a new `TextureDescriptor::array_layer_count`. The
existing cube map is the closest analog (it is already a layered
texture), so each backend follows the same path, except the layer count
is explicit rather than fixed at 6.

Backends:
- Metal maps the type to `MTLTextureType2DArray` and sets `arrayLength`.
- Vulkan maps it to a 2D-array image view (`e2DArray`) with the
descriptor's layer count.
- OpenGL ES allocates the array with `glTexImage3D` and fills layers
with `glTexSubImage3D`, gated on a new
`CapabilitiesGLES::SupportsTextureArray` that is true on ES 3.0+ (and
desktop GL 3.0+) and false on ES 2.0, where there is no array-texture
equivalent so callers fall back to a texture atlas.

Per-layer upload already worked through the existing `slice` parameter
on Metal and Vulkan.

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- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

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