Enable mustache-inspired string templates for `key_value_stores` and others

I'm building a Spin App that consists of multiple components:

# ...

[component.a]
source = "a/target/wasm32-wasi/release/a.wasm"
key_value_stores = ["data"]

[component.b]
source = "b/target/wasm32-wasi/release/b.wasm"
key_value_stores = ["data"]

# ...

Instead of providing the magic string (name of the desired key-value store) 4 times (twice as part of key_value_stores and once per component), I would want to use mustache-inspired string templates as illustrated here:

#...

[variables]
store = { default = "data" }

[component.a]
source = "a/target/wasm32-wasi/release/a.wasm"
key_value_stores = ["{{ store }}"]

[component.a.variables]
store = "{{ store }}"

[component.b]
source = "b/target/wasm32-wasi/release/b.wasm"
key_value_stores = ["{{ store }}"]

[component.b.variables]
store = "{{ store }}"

# ...

Review the TOML examples in the issue and trace how variables, key_value_stores, and component variables are currently represented and resolved. Done means a shared variable such as store can be used in those fields with mustache-inspired syntax, while preserving the existing configuration behavior.