[io] Add field user-defined metadata; Tweak API for tf.io.fromMemory() (

#1864)

FEATURE

- This is the first PR for adding support for user-defined metadata in model artifacts. Design doc has been circulated and discussed.
- Add the field `userDefinedMetadata` to `ModelArtifacts` and `ModelJSON`.
- Deprecate the old API of `tf.io.fromMemory()` which consisted of multiple arguments. The arguments are consolidated into on in the new API.
- Add unit tests.

Towards tensorflow/tfjs#1596

make version 1.2.6 (#1860)

Update tfjs-core to 1.2.5. (#1853)

INTERNAL

Update tfjs-core to 1.2.3. (#1842)

INTERNAL

Update tfjs-core to 1.2.2. (#1806)

INTERNAL

Update tfjs-core to 1.2.1. (#1784)

INTERNAL

Update tfjs-core to 1.2.0 (#1754)

INTERNAL

Update tfjs-core to 1.1.2. (#1726)

INTERNAL

Update tfjs-core to 1.1.1. (#1720)

INTERNAL

Update core to 1.1.0 (#1678)

INTERNAL