feat(secrets): Implement into_iter for SecretStore

[jonaro00]

Description of change

Title~
Came up in a discussion about secrets https://discord.com/channels/803236282088161321/1107140940759568384/1118625700824485928

How has this been tested? (if applicable)

Added test

[dteare]

For context for others who don't want to wade through the entire Discord thread: basically I'm wanting to use Rocket's outstanding figment feature that allows you to extract config values directly into structs without futzing around. For example, rocket.figment().extract::<OpenAiConfig>() gives me all I need to configure OpenAI without working with each field individually.

Rocket does this automatically based on environment variables. Shuttle Secrets (set via Secrets.toml) do not get injected into applications as environment variables, and so figment and others were sad.

The idea was to work around this limitation by iterating over all the secrets within SecretStore and call std::env::set_var for each one. SecretStore doesn't provide a convenient way to do this, hence Shuttle Hero @jonaro00 jumped into action with this MR.

[iulianbarbu]

LGTM! Thanks!