rust/detect: Add entropy support

rust/detect: Add entropy support

This commit adds
- Parser for the entropy keyword
- Calculation of content the Shannon entropy value

Issue: 4162

The entropy keyword syntax is the keyword entropy followed by options
and the entropy value for comparison.

The minimum entropy keyword specification is:
entropy: value <entropy-spec>

This results in the calculated entropy value being compared with
<entropy-spec> with the equality operator.

Calculated entropy values are between 0.0 and 8.0, inclusive.

A match occurs when the values and operator agree. This example matches
if the calculated and entropy value are the same.

When entropy keyword options are specified, all options and "value" must
be comma-separated. Options and value may be specified in any order.

Options have default values:
- bytes is equal to the current content length
- offset is 0
- comparison with value is equality

entropy: [bytes <byteval>] [offset <offsetval>] value <entropy-spec>

Using default values:
entropy: bytes 0, offset 0, value =<entropy-spec>

<entropy-spec> is: <operator> (see below) and a value, e.g., "< 4.1"

The following operators are available from the float crate:
    - =  (default): Match when calculated entropy value equals specified entropy value
    - <  Match when calculated entropy value is strictly less than specified entropy value
    - <= Match when calculated entropy value is less than or equal to specified entropy value
    - >  Match when calculated entropy value is strictly greater than specified entropy value
    - >= Match when calculated entropy value is greater than or equal to specified entropy value
    - != Match when calculated entropy value is not equal to specified entropy value
    - x-y Match when calculated entropy value is in the range, exclusive
    - !x-y Match when calculated entropy value is not in the range, exclusive