…option as text

The previous serde::Deserializer for ClientConfigArgs::from_env only implemented
deserialize_struct, walking a static field list against std::env::var_os. That cannot
support #[serde(flatten)], which grouping fields into thematic sub-structs later needs while
keeping today's environment variable names.

config and conf were both evaluated and rejected first: both lower-case an environment
variable's key before any case-conversion option runs, which cannot read ArmoniK's inherited
PascalCase convention (GrpcClient__CertPem) no matter how they are configured, confirmed by
reproducing the failure against their vendored source. figment's Env provider, used as
Env::prefixed(prefix).lowercase(false), preserves that casing, verified against its own
source and a scratch project before adopting it.

figment's Env provider parses each variable's raw text into a typed value before serde ever
sees it: a bare 3 becomes an integer, true becomes a boolean, and a value entirely enclosed
in brackets or braces becomes a list or object. Every field of ClientConfigArgs is meant to
carry plain text regardless of source, so a text/secret_text deserialize_with pair, applied
to every field, coerces a scalar back to its own spelling; a value that is itself a full
bracketed list is refused with a message naming its escape hatch, wrapping it in a literal
pair of double quotes.

The three boolean fields become String for the same reason: a deserialize_with shim's
signature never carries the field it was called for, so the vocabulary wider than
true/false is now interpreted once, in from_config_args, which is what can still name the
option on a bad value (a new ConfigError::InvalidBool).

ClientConfig::from_env keeps its old zero-argument signature for now, hardcoding
GrpcClient__ as the prefix: making it a parameter of its own, and retiring the method
entirely in favour of composing HttpConfigArgs::from_env in the armonik crate, is the next
commit's concern.