Summary:
## Why this diff
we want hostname prefix to support targeting configs at clients in corp ("corp" means laptop, labs, and other machines that are not in "prod" datacenters), like FRL machines, that don't support our existing tier mechanism.
## Changes
* Extract hostname prefix in `dynamicconfig.rs` and add a getter function `hostname_prefix()` for it.
*A hostname prefix only consists of alphabetical letters and dashes, which is followed by one or more digits in the hostname. If no valid match, the prefix is set to the empty string.*
* Use `gen.hostname_prefix()` in the `evaluate()` fn inside `mod.rs` to check the generator's prefix against a list of given prefixes.
* Copy changes from `configerator/source/scm/hg/hgclientconf/hgclient.thrift` to `fbsource/fbcode/configerator/structs/scm/hg/hgclientconf/hgclient.thrift`.
* Rebuild in `eden/scm/`.
Reviewed By: DurhamG
Differential Revision: D26706686
fbshipit-source-id: 725506a1c1f0983e981b0b3f3993c7c14510b1db
Any native code (C/C++/Rust) that Mercurial (either core or extensions)
depends on should go here. Python code, or native code that depends on
Python code (e.g. #include <Python.h> or use cpython) is disallowed.
As we start to convert more of Mercurial into Rust, and write new paths
entrirely in native code, we'll want to limit our dependency on Python, which is
why this barrier exists.
See also hgext/extlib/README.md, mercurial/cext/README.mb.
How do I choose between lib and extlib (and cext)?
If your code is native and doesn't depend on Python (awesome!), it goes here.
Otherwise, put it in hgext/extlib (if it's only used by extensions) or
mercurial/cext (if it's used by extensions or core).
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