mutual-tls: Add support for Meta certificates

Meta Inc's client certificates use an internal Subject CN format
which AFAIK is specific to Meta and don't adhere to a known standard:

  Subject: CN = <type>:<entity>/<hostname>

Commit adds the `mutual-tls-common-name-parsing=meta` option to, on
Meta builds, parse the Subject CN field and map either the <entity>
to a local user.

The <type> field determines what kind of client identity the cert
represents. Only type="user" is supported for now with <entity> being
the unixname of a Meta employee. For example, the Subject CN string
below maps to a local BMC user named "kawmarco":

   Subject CN =  "user:kawmarco/dev123.facebook.com"

Tested: Unit tests, built and tested on romulus using the script below:
https://gist.github.com/kawmarco/87170a8250020023d913ed5f7ed5c01f

Flags used in meta-ibm/meta-romulus/conf/layer.conf :
```
-Dbmcweb-logging='enabled'
-Dmutual-tls-common-name-parsing='meta'
```

Change-Id: I35ee9b92d163ce56815a5bd9cce5296ba1a44eef
Signed-off-by: Marco Kawajiri <kawajiri@meta.com>
diff --git a/config/meson.build b/config/meson.build
index 10ddb2d..1ba34af 100644
--- a/config/meson.build
+++ b/config/meson.build
@@ -20,6 +20,8 @@
 conf_data.set10('BMCWEB_ENABLE_MULTI_HOST', enable_multi_host.allowed())
 enable_http2 = get_option('experimental-http2')
 conf_data.set10('BMCWEB_ENABLE_HTTP2', enable_http2.allowed())
+conf_data.set10('BMCWEB_ENABLE_MTLS_COMMON_NAME_PARSING_META', get_option('mutual-tls-common-name-parsing') == 'meta')
+
 
 # Logging level
 loglvlopt = get_option('bmcweb-logging')
@@ -59,4 +61,3 @@
   install_dir: '/etc/pam.d/',
   rename: 'webserver',
 )
-