feat: support complex map types

[SkArchon]

This PR adds support for complex map types, enabling users to configure environment variables that map to Go map structures. References #361 (Note: This is the same as this PR #362, The source branch was changed thus the PR was reopened separately)

This is useful when users might have a structure such as

type SubEntry struct {
	Str string `env:"DAT_STR"`
	Num int    `env:"DAT_NUM"`
}
type ComplexConfig struct {
	Bar map[string]Test `envPrefix:"BAR_"`
}

Where the key to access the SubEntry struct can be dynamic.

We need to figure out the key of the map, the algorithm for this essentially reads as follows

Traverse the struct of the map and find out the sub keys it has and use the matching key to trim the suffix of the map environment variable to find the key.

This results in the following restrictions, if a user was to use "A map of structs inside a map of structs", it becomes harder to figure out the key, thus in these cases instead of trimming the suffix, we split the string using the envPrefix of the sub (inner) map of the last recurring (rightmost) substring. Users should be more aware with the naming of the tags (env and envPrefix) and should make them unique, when there are nested maps involved.

Test cases with examples for these cases are available, e.g. :- https://github.com/caarlos0/env/pull/362/files#diff-19e9f6a4b37808acd7acef7094d89f6f9ca8e7c6409ab9b583fa3fac69000ec6R2529

We also ignore env tag options like ,init etc on the map field itself, this is similar to how slices of structs behaves

[SkArchon]

@caarlos0 Do let me know your thoughts on this proposal