…var (#1205)

### What
Currently if you want v3-engine to accept metadata that is only a
partial supergraph (ie missing some subgraphs), you must set the
PARTIAL_SUPERGRAPH env var. Builds sent to MBS get sent to a special
endpoints `/validate/partial` and `/build/partial` that runs the engine
build process with that configuration option set.

This results in a terrible user experience for local builds, because MBS
will accept a partial supergraph and yield build artifacts, but
v3-engine will refuse to run them unless you set that env var.

This PR removes that env var and creates a new OpenDD flag called
`allow_partial_supergraph`. When MBS's `/validate/partial` endpoint is
used, that flag is set in the build artifacts. v3-engine then looks at
that flag to enable partial supergraph mode. This means a
`/build/partial` build via MBS just works when you run it locally via
v3-engine.

### How

`metadata_resolve`'s `Configuration.allow_unknown_subgraphs` has been
removed, and `metadata_resolve` now looks at OpenDD's new
`Flags.allow_partial_supergraph` instead.

In MBS, usage of `ValidationMode` (the enum that enables partial builds)
previously used to set `Configuration.allow_unknown_subgraphs`; now it
is used in `compute_open_dds_flags` in order to set
`Flags.allow_partial_supergraph`.

The existing metadata_resolve test that tested partial supergraphs has
been modified to use the flags rather than the removed configuration
option
(`crates/metadata-resolve/tests/passing/missing_subgraph_when_ignoring_unknown_subgraphs`).

A new `metadata_resolve` test has been added that checks that comparable
relationships in `BooleanExpressionType` properly respects the
`allow_partial_subgraphs` flag (this functionality was added in #1182).

V3_GIT_ORIGIN_REV_ID: 2c984eb791263a1fb0606c6c44a2a1ae4a5e7370