Wrong bitwise alignment expected for LPM keys when they are not multiples of 8 bits in size

[jafingerhut]

The PI library (and behavioral-model repo, which has its own separate issue to track this) expect LPM keys to always be "left aligned" in an integer number of 8-bit bytes.

This matches what the P4Runtime API specification says, when the LPM key fields are a multiple of 8 bits wide.

However, when an LPM key field has a bit width W that is not a multiple of 8 bits wide, the P4Runtime specification requires that the W bits are in the least significant W bits of the minimum number of bytes required to hold W bits.

To have a more visual example:

# What the PI library expects today for an LPM key field with width W=20 bits, where 0
# bits represent bit positions that are outside of the LPM key value, and 1 bits represent
# bit positions that are part of the LPM key value.
1111_1111 1111_1111 1111_0000
^^^^^^^^^ most significant byte
                    ^^^^^^^^^ least significant byte

# What the P4Runtime specification says about where the bits should be for such a field:
0000_1111 1111_1111 1111_1111

Correcting this is a small change to the PI library code, but is likely to be a bit more changes within the behavioral-model repository code.