Capture the binding metadata CF sends

[norman-abramovitz]

Closes #25.

The actual gap

The literal question in #25 — "can I read the user-provided env?" — was already
answered by existing behavior:

• User-provided service instances arrive in VCAP_SERVICES under the
  user-provided label, so Services.WithLabel("user-provided"), WithName
  and WithTag have always resolved them.
• Env vars set with cf set-env are ordinary process environment, already
  returned by CurrentEnv().

What was genuinely broken is next door: VCAP_SERVICES carries eleven fields
per binding object and the decoder kept six, silently discarding the rest.

Field	Before	After
name, label, tags, plan, credentials, volume_mounts	kept	kept
syslog_drain_url	dropped	SyslogDrainURL
instance_guid, instance_name	dropped	InstanceGUID, InstanceName
binding_guid, binding_name	dropped	BindingGUID, BindingName

syslog_drain_url is the one a user-provided instance depends on — it is what
cf cups -l sets. And instance_name matters because name holds the binding
name when a binding has one, so without it there was no way to recover the
instance name for a named binding.

Implementation note

mapstructure does not map snake_case onto CamelCase by itself — verified by
adding the fields untagged first and watching four of the five stay empty. Each
field therefore carries an explicit tag, as volume_mounts already did.

The Name and Label doc comments were corrected while here: Name is the
binding name if the binding has one and otherwise the instance name, and Label
is the service offering name, which is user-provided for a user-provided
instance.

API impact

gorelease -base=v1.20.0:

## compatible changes
Service.BindingGUID: added
Service.BindingName: added
Service.InstanceGUID: added
Service.InstanceName: added
Service.SyslogDrainURL: added

# summary
Suggested version: v1.21.0

Purely additive.

Verification

• make check clean, make test-race passes
• coverage 90.9% -> 91.0%
• new tests cover a fully-populated user-provided binding and a plain binding
  where CF omits every new field

Also carries the changelog.d sweep for the fragments v1.20.0 consumed.