Fixes #463. Expose LLDP neighbors on EthernetInterface

[DavidVentura]

Description

This change adds a new function: get_lldp_neighbors to EthernetInterface, which returns a list of LLDP neighbors from the device.

Motivation and Context

LLDP is very useful to determine whether patching was carried out correctly, see #463

How Has This Been Tested?

I am testing this on a PA-850.

Lint rules from the contributing guidelines were broken prior to this change

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes if appropriate.
• All new and existing tests passed.

I have not added tests, I couldn't find a test for get_counters to base my test on, is there a similar test I can use as an example?

[welcome-to-palo-alto-networks]

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[shinmog]

@btorresgil

Does this feel like it should be in an opstate namespace...?