Add saving wallet config with `bdk-cli wallet config` #812

Job	Run time
Build and test (stable, --no-default-features)	1m 16s
Build and test (stable, --features default)	1m 6s
Build and test (stable, --all-features)	1m 59s
Build and test (1.75.0, --features default)	1m 31s
Rust fmt	6s
Build and test (1.75.0, --no-default-features)	1m 21s
Build and test (1.75.0, --all-features)	2m 13s
	9m 32s

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