Add singleValueMapping option

[plumdog]

To allow, eg:

# secrets.yaml, a sops encrypted file with plaintext contents
a:
  b: 'c'

new SopsSecretsManager(this, 'MyApiKey', {
    path: './secrets.yaml',
    secret: mySecret,
    singleValueMapping: {
        path: ['a', 'b'],
    },
});

would result in mySecret having contents c.

[sonarcloud]

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34.8% Duplication

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