Add saving wallet config with `bdk-cli wallet config` #813

Job	Run time
Build and test (stable, --no-default-features)	26s
Build and test (stable, --all-features)	2m 1s
Rust fmt	7s
Build and test (1.75.0, --all-features)	2m 8s
Build and test (1.75.0, --no-default-features)	28s
Build and test (1.75.0, --features default)	25s
Build and test (stable, --features default)	21s
	5m 56s

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