Add saving wallet config with `bdk-cli wallet config` #821

Job	Run time
Rust fmt	16s
Build and test (stable, --features default)	1m 15s
Build and test (1.75.0, --features default)	1m 16s
Build and test (stable, --all-features)	1m 23s
Build and test (1.75.0, --all-features)	1m 24s
Build and test (1.75.0, --no-default-features)	1m 16s
Build and test (stable, --no-default-features)	1m 8s
	7m 58s

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