This change adds a `.bazelversion` file which allows `bazelisk` users to
use the expected version of `bazel` when building. The version of bazel
specified matches that of the one used in Github Actions currently.

This is a small quality-of-life improvement, as otherwise the default of
"latest release" will download bazel 7, fail to build, and create
bzlmod-related files in the process.

This change adds a config option for `RoutingBlobAccess`, which is
intended to hold all possible ways to rewrite instance names.

While it is possible to affect instance name by (ab)using
`DemultiplexingBlobAccess`, there are some sharp edges (e.g. using it to
add an instance name may not work as expected, as not only must the
instance name be valid on the inner blobstore, but it must be valid once
returned upwards as well). `RoutingBlobAccess` will attempt to fix that
by offering easier-to-use semantics around instance name rewriting.